National estimates indicate that roughly 70% of children in foster care have one or more siblings also in care. Exact percentages vary by state, but several data sources suggest that about 50% of early adolescent foster youth are placed apart from one or more of their siblings. Relationships between these siblings may be critical in providing them a sense of connection and emotional continuity as they are removed from much that is familiar (e.g. their home, parents, and friends). Historically, efforts to strengthen ties of foster children with their families have focused on adults (e.g., biological parents, grandparents). Limited research exists on the relationships of siblings in foster care. The central aim of the proposed study is to evaluate the impact of the Sibling plus Parent Management Training (SPMT) intervention as compared to Parent Management Training only (PMT) on key constructs of sibling relationship quality, mental health, academic success, and quality of life for youth in foster care. Conducted in partnership with the Oregon DHS Foster Care program, 240 sibling dyads and their foster parents will be enrolled in six cohorts. Siblings may live together or in separate placements. Dyads will consist of (1) a target youth in care that is 11-15 years of age, and (2) a younger sibling in care who is 7-15 years of age and within 4 years of age of the target youth. Sibling dyads will be matched as living together or living apart; the matched dyads will be randomly assigned to either the SPMT or PMT-only group, with all study-enrolled foster parents receiving PMT intervention components. The SPMT intervention includes a sibling component as well as foster parent PMT. The sibling intervention component includes eight cognitive behavioral sessions of skills learning/practice, and four community activities planned by siblings with their interventionist coaches. For foster parents, there is a 4-session PMT curriculum emphasizing skills learning and practice with their study-enrolled foster child. Additional sessions are available to foster parents on request. Foster parents will also be able to access and troubleshoot PMT materials and strategies via weekly staff check-in calls, and the project website. Major wave assessments will be conducted at baseline, intervention termination (6 months), follow-along1 (6-month post-intervention) and follow-along2 (12-month post-intervention). Brief, bi-monthly phone interviews for youth and foster parents will collect service utilization data and global ratings of outcome constructs for use in growth-modeling analyses. Hierarchical Linear Models (HLM) and multiple regression analyses will be used to test (1) the prediction that SPMT siblings will improve more than PMT siblings on key outcomes, and (2) whether intervention efficacy varies by sibling placement (together vs. apart) and participant characteristics such as race, gender, and disability.